That should be the amplitude if I am not mistaken, basically it shows the +/- tolerance for its height above and below its relative 0.

For example, and I could be wrong, if you do

velocity.y = Math.cos(x / 50) * 50;

then the waves will be close together, but the height of the wave will be +50px or -50px for its relative 0 (the Y it initially started on)

Changing the value inside the cos() determines the wave length

Edit: in case you didn't read the article on Sine Waves:

A, the amplitude, is the peak deviation of the function from its center position.

ω, the angular frequency, specifies how many oscillations occur in a unit time interval, in radians per second

φ, the phase, specifies where in its cycle the oscillation begins at t = 0.

When the phase is non-zero, the entire waveform appears to be shifted in time by the amount φ/ω seconds. A negative value represents a delay, and a positive value represents a "head-start".

Though, this is a bit more complicated than what you are doing, but it basically shows you more in depth what the values are when doing a sine wave